New test speeds search for resistant bananas

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Cuban researchers have developed a quick and easy method for assessing the susceptibility of banana plants to a serious fungal infection known as Panama disease. Their findings could make it easier to select resistant strains of banana for wide-scale planting.

In recent years, Panama disease has had a major impact on banana yields in Africa, Asia and Australia, and is expected to spread to Latin America and the Caribbean. The issue has been at a centre of a controversy over whether the disease could eventually lead to the extinction of bananas.

The fungus that causes the disease, known as Fusarium oxysporum, establishes itself in the soil and is virtually impossible to control with chemicals.

The current method of detecting which banana strains are susceptible to the disease is time-consuming and laborious. It involves infecting soil with the fungus, waiting for the plant to grow, and then cutting the plant at the base of the stem to evaluate the extent of internal damage.

"This destroys the plant, making continued study [of the same plant] impossible", says Barbarita Companioni of the Bioplant Centre, University of Ciego de Avila, Cuba, one of the team that carried out the research.

The new test takes just a few days to get results, and can be carried out on a single leaf. The fungus is grown in the laboratory and applied to tiny perforations in harvested banana leaves. After 48 hours, lesions of around 18 mm in length can be seen in the leaves of susceptible plants, while leaves from resistant plants have much smaller lesions of less than 7 mm.

The Cuban Research Institute for Tropical Food Crops in Villa Clara is already using the new test in its programme of banana genetic improvement. This seeks to develop, through both traditional breeding techniques and genetic modification, banana and plantain hybrids that are resistant to Panama and other diseases.

Ninety nine per cent of bananas found in Western supermarkets belong to a single variety, Cavendish, a sterile clone that is propagated by cuttings. The variety was initially adopted in the 1960s, when Panama disease forced banana growers around the world to abandon previous varieties. But now Cavendish bananas are being infected with a new virulent strain of the fungus, called race 4, making it essential that new resistant strains are developed.

The Swedish-based International Foundation for Science and the Cuban Ministry of Science, Technology and the Environment, which jointly funded the research, are now supporting the commercial development of the test.

"Our test has had an impact because it has meant saving resources and time and so helped to identify varieties in a short period of time," conclude project leaders José Carlos Lorenzo and Ramón Santos.